diff --git a/chrome/content/zotero/xpcom/data/dataObjects.js b/chrome/content/zotero/xpcom/data/dataObjects.js
index 9167b8421..b701142a1 100644
--- a/chrome/content/zotero/xpcom/data/dataObjects.js
+++ b/chrome/content/zotero/xpcom/data/dataObjects.js
@@ -275,6 +275,9 @@ Zotero.DataObjects = function (object, objectPlural, id, table) {
 	
 	
 	this.getIDFromLibraryAndKey = function (libraryID, key) {
+		if (libraryID === null) {
+			throw new Error("libraryID cannot be NULL (did you mean 0?)");
+		}
 		return (this._objectIDs[libraryID] && this._objectIDs[libraryID][key])
 			? this._objectIDs[libraryID][key] : false;
 	}